You do not need a cartridge, although you can still buy these on eBay. Each cartridge holds, IIRC, twice as much as the internal memory.

You can change the sounds from the front panel, but unless you have a good understanding of FM synthesis, I do not recommend this. Get a MIDI cable to hook your DX-7 up to your computer, and locate a suitable patch editor. Google, there are many out there. Then play.

You will also find tonnes of pre-programmed patches online. But to get the most out of this unit, you need to learn how to program it yourself.

The best book on this is Howard Massey's "The Complete DX7". Unfortunately, this is now out of print and is very pricey on the used market...upwards of $90 U$S. But as Wes noted above, programming FM synthesis is a daunting task in light of the thousands of free DX patches now available on the internet...just Google: "free DX7 patches". If you really do want to wade in, you'll need an editor, try this one, free for 30 days then only $20. www.fm-alive.com